Über dieses Buch

Experimental and Applied Mechanics, Volume 6: Proceedings of the 2014 Annual Conference on Experimental and Applied Mechanics, the sixth volume of eight from the Conference, brings together contributions to important areas of research and engineering. The collection presents early findings and case studies on a wide range of topics, including:

Advances in Residual Stress Measurement Methods

Residual Stress Effects on Material Performance

Inverse Problems and Hybrid Techniques

Thermoelastic Stress Analysis

Infrared Techniques

Research in Progress

Applications in Experimental Mechanics

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Inhaltsverzeichnis

Frontmatter

Structural heath monitoring (SHM) can provide an estimate of the state of damage in a structure, and of the remaining useful life of that structure. The work presented here is an investigation of a proposed new SHM technique for composite structures composed of carbon-fiber-reinforced-polymers (CFRP). Electrical impedance spectroscopy (EIS) is employed to estimate the damage state of the composite. No modification to current CFRP processing methods is required, nor is the proposed technique invasive or destructive. EIS interfaces can be either permanently attached or temporarily connected. We hypothesize that EIS has the potential to be more sensitive and selective for damage detection by using a full complex-plane analysis, considering both impedance magnitude and phase angle. This is in contrast to electrical SHM approaches employing resistance measurement, the real component of impedance, which ignores phase angle and reactance information. In order to test our hypothesis we implemented three different experiments to evaluate the effectiveness of the EIS technique: (1) specimen load sensitivity; (2) specimen damage sensitivity; and (3) specimen fatigue sensitivity. Multiple electrical interrogation paths through the specimen are considered.

A novel experimental investigation is presented of thermally and stress induced transformation behaviour of a Polycrystalline NiTi Shape Memory Alloy (SMA) plate for flexural-type applications: In situ techniques are employed to allow simultaneous macroscopic and microstructural observation of the SMA in a 4-point flexural test. Forming part of a wider research towards realising a NiTi SMA Variable Stator Vane assembly for the gas turbine engine, the study explores variables critical to flexural-type morphing NiTi structures: (1) temperature; (2) strain; and (3) cyclic loading. It builds a relationship between the macro and micro response of the SMA under these key variables and lends critical implications for the future understanding and modelling of shape memory alloy behaviour for all morphing applications. This paper presents the methodological aspects of this study.

With growing interest in nanotechnology, the manufacturing industries for Micro-Electro-Mechanical Systems (MEMS) and Nano-Electro-Mechanical-Systems (NEMS) are constantly thriving towards extraordinary precision in the machining and etching tools. It is a common practice, during manufacturing, a set of instructions are provided to a manufacturing tool, by actuating them at certain frequencies to perform their respective tasks. Every different task (e.g. cutting nano-channels, drilling micro-holes, nano-welding etc.) has unique instruction with unique frequency input. In such cases, other than the desired frequencies, remaining possible frequencies in the system needs to be filtered or stopped. It is extremely challenging to avoid system noises electronically and select or actuate specific frequencies. Hence, in a noisy environment (e.g. fluctuation of temperature, external vibration etc.) it is extremely difficult to provide a unique frequency to a tool to perform a task precisely without having an uncertainty. In this study, we intend to propose a mechanical model to precisely sense, pass and actuate desired frequencies and filter unwanted input frequencies, which in turn will result a mechanical pass band sensor. Traditionally researches are interested in stopping undesired frequencies to pass desired frequencies through local resonance phenomena. However, if only certain frequencies are required, it is extremely difficult to filter all unnecessary frequencies by creating frequency band gaps. Hence, in this effort, bio-inspired logistic, adopting local resonator physics is employed by extracting the benefit of unique frequency sensing, mechanically. Human cochlea senses only sonic (20 Hz to 20 kHz) frequencies by filtering all other frequencies in the environment. Basilar membrane is the principal component of the human cochlea with logarithmically decreasing stiffness from its basal to apical end. Basilar membrane composed of series of thin micro beams attached to each other, where each beam holds unique bending rigidity and hence, capable of resonating at a particular sonic frequency. Similarly, in this study, to replicate the functionality of a basilar membrane, a discrete mass-in-mass (DMM) metamaterial model is proposed while using a complete different physics of local resonance. It is hypothesized that, systematic arrangement of such DMM cells can select of band of frequencies, predictively.

Dynamic equations for an isotropic spherical shell are derived by using a series expansion technique. The displacement field is split into a scalar (radial) part and a vector (tangential) part. Surface differential operators are introduced to decrease the length of the shell equations. The starting point is a power series expansion of the displacement components in the thickness coordinate relative to the mid-surface of the shell. By using the expansions of the displacement components, the three-dimensional elastodynamic equations yield a set of recursion relations among the expansion functions that can be used to eliminate all but the four of lowest order and to express higher order expansion functions in terms of these of lowest orders. Applying the boundary conditions on the surfaces of the spherical shell and eliminating all but the four lowest order expansion functions give the shell equations as a power series in the shell thickness. After lengthy manipulations, the final four shell equations are obtained in a more compact form which can be represented explicitly in terms of the surface differential operators. The method is believed to be asymptotically correct to any order. The eigenfrequencies are compared to exact three-dimensional theory and membrane theory.

Recent experiments provided evidence of piezonuclear reactions occurring in condensed matter during fracture of solids, cavitation of liquids, and electrolysis. These experiments were characterized by significant neutron and alpha particle emissions, together with appreciable variations in the chemical composition. A mechanical reason for the so-called Cold Nuclear Fusion was recently proposed by the authors. The hydrogen embrittlement due to H atoms produced by the electrolysis plays an essential role for the observed microcracking in the electrode host metals (Pd, Ni, Fe, etc.). Consequently, our hypothesis is that piezonuclear fission reactions may occur in correspondence to the microcrack formation. In order to confirm the first results obtained by Co-Cr and Ni-Fe electrodes, electrolytic tests have been conducted using 100 % Pd at the cathode. As a result, relevant compositional changes and traces of elements previously absent have been observed on the Pd and Ni-Fe electrodes after the experiments and significant neutron emissions were observed during the test.

A bi-stable energy harvester utilizing PVDF strips driven via two torque arms with end masses and pseudo pinned in the middle is evaluated. A sinusoidal acceleration is applied to the base of the device with varying frequencies and magnitudes while the compression of the center beam is achieved by applying a small displacement to the center beam. Frequency sweeps will be done forwards as well as backwards to evaluate hysteresis performance. Peak voltages, natural frequencies, snap-through acceleration values, static actuation displacement values, and material properties for unknowns are derived experimentally.

While many parametric values such as beam length, compliance arm length, and proof mass can be varied, the focus of this study is on the effects of the compliance arm width on bi-stability switching and energy harvesting potential. For vibration-based energy harvesting, performance parameters such as power generated, power density, frequency broadening, frequency shifting, and optimal load impedance will be quantified. Results show that wider compliance arms decrease buckling amplitude, but increase the bi-stability switching regime and the overall power production. Current data also indicates that an optimal compression load exists for a given acceleration value.

The latest release of LS-DYNA includes a multi-physics solver that combines computational fluid dynamics and structural solvers. This capability is a new computational design tool for the automotive industry. One of the simplest ways to reduce weight and increase fuel efficiency is to trim unnecessary weight from the body panels which comprise the vehicle. However, body panels that are made too thin are susceptible to a phenomenon known as oil-canning under loads such as those encountered from typical automotive air dryers. Oil-canning is a complex phenomenon that can result in permanent deformation or the panel can snap back. Oil-canning is to be avoided, even if temporary, for customer satisfaction reasons. An experimental program is presented where automotive roof panels are placed in a custom test rig and loaded with a high velocity air jet to replicate the oil-canning phenomenon. Flow characterization is performed using an array of piezo-electric pressure sensors. Panel deformation is measured using three-dimensional digital image correlation. Experimental data will be used to determine the validity of the multi-physics solver as an engineering design tool.

The dependence of the fundamental frequency on the axial load in slender beams subject to imposed axial end displacements was experimentally investigated. The considered specimens presented different geometrical imperfections (initial curvatures), and were tested in two different constraint conditions (hinged–hinged and hinged-clamped). The natural frequencies were extracted in both conditions of forced and free vibration, using an electromagnet and a laser displacement transducer. In addition, the responses observed during the experiments, for the hinged–hinged case, were reproduced by numerical simulations, obtaining a good agreement between numerical and experimental results.

Microgravity experimentation enables new materials to be developed and traditional materials to be improved, which can’t be completed under terrestrial conditions. Recent developments on Heavy Metal Fluoride Glasses (HMFG’s) have shown that, when heated, there is a crystallization dependency on gravity. HMFG’s have the potential for optical transmission from 0.3 μm in the UV to 7 μm in the IR region, enabling fiber optic applications such as fiber amplifiers, radiometry, and mid-IR laser technology for surgery, drilling and cutting. The problem of devitrification from heat processing prevents this material from achieving its theoretical transmission range. Past researchers have shown that crystallization of HMFG’s is suppressed in microgravity and enhanced in hyper-gravity, however further investigation is still needed for a determination of this phenomenon. In this study, a HMFG heating and quenching testing apparatus was characterized and developed for microgravity and hyper-gravity testing. The testing apparatus was developed and characterized for use on a parabolic aircraft that provides a microgravity and hyper-gravity environment for experimental testing. The apparatus was successful in processing HMFG’s, which produced crystalline and non-crystalline glasses for future studies.

Phase-shifting technique is widely used in phase detection field. With the aid of this technique, hundreds to thousands of wavelength accuracy result can be achieved. Further calculating the grabbed phase-stepping frames yields the phase map, which is in wrapped format and has to be further treated to convert into a continuous (i.e., an unwrapped) mode. Different algorithms—path-dependent or path-independent, temporal or spatial, point-by-point or regional, noise-sensitive or noise-immune, etc., have been proposed for solving problems in different applications. In present study, graphic processing unit (GPU), which has been developing rapidly in recent years, is utilized in addition to shorten and accelerate the processing time needed. By the aid of GPU parallel processing technique, the retrieving work is much more time effective. Since temporal phase unwrapping deals wrapped data from load-stepping or wavelength-stepping basis, it can be easily benefit from the parallel processing of GPU. Experimental work of photoelasticity is verified by the present study.

In this paper, the non-linear elastodynamics of a flat plate subjected to a low velocity foreign body impact is studied experimentally. The work is based on a central hypothesis that in addition to identifying the impact locations, the material properties of the foreign objects can be classified using acousto-ultrasonic signals. A novel cluster of thin piezoelectric sensors is proposed and a carefully formulated dominant frequency approach is studied to investigate the nonlinearities. Such nonlinearities with their highest resolution are quantified with the proposed Theodorus spiral configuration of the sensors (TSSC). It is found that the frequency and speed of the guided wave generated in the plate can be quantized based on the impactor's relationship with the plate, i.e. the wave speed and the impactor's mechanical properties are coupled. In this work, in order to characterize the impact location and mechanical properties of impactors, nonlinear transient phenomenon is empirically studied to decouple the understanding using the dominant frequency band (DFB) and lag coefficients of the acousto-ultrasonic signals through TSSC. Next the understanding was correlated with the elastic modulus of the impactor to predict transmitted force histories.

Hole-drilling measurements of residual stresses are traditionally made on materials that are either very thick or very thin compared with the hole diameter. The calibration constants needed to evaluate the local residual stresses from the measured strain data are well established for these two extreme cases. However, the calibration constants for a material with finite thickness between the extremes cannot be determined by simple interpolations because of the occurrence of local bending effects not present at either extreme. An analytical model is presented of the bending around a drilled hole in a finite thickness material and a practical procedure is proposed to evaluate the corresponding hole-drilling calibration constants.

Normally, the residual stresses as a result of shot peening, include compressive surface stresses on the treated side and tensile stresses on the subsurface. Residual stresses from shot peening of Bulk Metallic Glasses have been beneficial for improving their plasticity in compression. Recently, significant residual stresses were observed in abrasive treated metallic glass ribbons using a process similar to shot peening. Significant thermal residual stresses have also been predicted in amorphous metals, from the rapid quenching needed to retain their structure. Early measurements used neutron diffraction to identify the thermal stresses in metallic glasses. Peening using glass beads leads to curvature as a result of surface stresses. Optical measurements show a reduction in ribbon thickness. XRD measurements on the abrasive treated metallic glass also showed shifts of the broad amorphous diffraction hump compared to untreated ribbons, indicative of the change in residual strain. The resolution of X-ray strain measurements on amorphous metals and the relaxation of thermal residual stresses were considered for validating the prediction.

X-ray diffraction and hole-drilling methods are applied to measure the residual stresses in a turbo charge compress wheel made of aluminum wrought alloys for finite element model validation. Aluminum wrought alloys are usually subjected to heat treatment which includes quenching after solution treatment to improve aging responses and mechanical properties. Rapid quenching can lead to high residual stress and severe distortion which significantly affect dimension stability, functionality and particularly performance of the product. A finite element based approach was developed by coupling a nodal-based transient heat transfer algorithm with material thermo-viscoplastic constitutive model, to model residual stress and distortion during heat treatment for robust product design and durability assurance. The comparison shows that hole-drilling residual stress measurements provide more accurate and reliable results than X-ray diffraction for this particular part and material. A good agreement between residual stress measurement and FEA prediction has demonstrated that the integrated residual stress model is robust in predicting residual stresses and optimizing heat treatment of aluminum wrought alloys.

The present paper is aimed at investigating the effect of shot peening on the fatigue behavior of Al-7075-T651 samples carrying different types of notches. The Wöhler S–N curves were determined by pulsating bending for the different experimental conditions. A different improvement of the fatigue strength was found, i.e. a different effectiveness of the treatment for different notch geometries: the more critical notches received the larger beneficial effect by shot peening treatment in terms of reduction of the notch fatigue sensitivity and increment of the notch fatigue strength. The fatigue improvements with respect to the unpeened condition were discussed accounting for the residual stress effects. The residual stress field ahead of the notch root was evaluated by means of a numerical technique, making use of XRD measurements on the plain peened specimens. The numerical predictions were then compared with the results of XRD measurements conducted in the vicinity of the notch root of the peened notched samples.

Residual stress measurement by interpreting diffraction rings is well developed. The ring is produced by many grains contributing individual diffraction spots, each from their local stress environment. Local stresses limit the bulk strain resolution of X-ray stress methods. The local stresses can be so large, traditional methods fail to produce meaningful results. However, if the local stress environment could be understood, it provides additional value for measuring and predicting material behavior. Experimental determination of the internal stress for one grain from a ring has several challenges. In some special cases, these have been overcome using synchrotron radiation. Here, using a Bruker D8 laboratory X-ray diffractometer and a 2D Hi-star detector, a method of sensing and analyzing X-ray diffraction cones in three dimensions was introduced. After a certain sample detector distance, individual grains can be resolved in spots on the ring. The method requires collecting a sequence of 2D frames at increasing sample to detector distances. The entire three-dimensional X-ray diffraction pattern (XRD3) could be used to determine the average 2θ ring position. This allows new types of strain measurements. Other applications for tracking spots from grains are in development.

The ring core method is a well-known technique for residual stress measuring. It consists of milling a circular ring around the point of interest and measuring the surface deformations of the core. The method is more sensitive than hole drilling, but its sensitivity decreases with depth to become null when ring depth is equal to one third of the diameter. To overcome this problem, in literature an incremental version of the technique has been proposed consisting of removing the core, re-installing the strain gauge rosette and re-performing the measurement. Although the idea is interesting, its practical implementation is quite difficult, in particular re-installing the rosette is almost impossible when depth becomes significant, thus the incremental measurement is never performed.

In this work we propose to replace the strain gauge rosette with an optical technique. In this way the incremental approach becomes viable, even though, depending on the optical technique used, some practical problems have to be addressed.

This article presents a methodology to optimize the design of a mechanical test to characterize all the material stiffness parameters of a PVC foam material in one single test. The two main experimental techniques used in this study are Digital Image Correlation and The Virtual Fields Method. The actual image recording process was mimicked by numerically generating a series of deformed synthetic images. Then the whole measurement procedure was simulated by processing the synthetic images with DIC and VFM routines. This procedure was used to predict the uncertainty of the measurements (systematic and random errors) by including the most significant parameters in actual experiments. By using these parameters as design variables and defining several error functions, the optimization study was performed to minimize the uncertainty of the identification and select the optimal test parameters. The simulated confidence intervals of the identified parameters were defined based on the predicted systematic and random errors. The simulated experimental results also indicated that averaging multiple images could lead to a significant reduction of the random error. The experimental validation was conducted using the optimized test parameters obtained from the numerical study. The results displayed a very good consistency with the simulation.

In the previous study, the conventional Ibrahim time-domain method (ITD) using free-decay responses of structures has been extensively used in the modal-identification analysis, however, which is only applicable to identify the modal parameters of an under-damped structure. In the present paper, we propose a theoretical modification for ITD method, and extend the ITD method for modal identification of over-damped structural systems. The eigenvectors of the system matrix used in extended ITD method corresponding to the vibrating modes of a structural system in pair are sorted through the assurance index proposed in this paper based on the theory of structural dynamics. Numerical simulations confirm the validity of the proposed method for modal identification of over-damped structural systems.

Non-destructive testing (NDT) is critical for many precision industries because it can provide important information about the structural health of critical components and systems. In addition, NDT can also identify situations that could potentially lead to critical failures. Specifically, NDT by optical methods have become popular because of their non-contact and non-invasive nature. Shearography is a high-resolution optical NDT method for identification and characterization of structural defects in components and has gained wide acceptance over the years; however, in practice, application of laser shearography for structural health monitoring requires loading the sample, which in some cases is not applicable or requires an experienced operator to properly perform structural testing. In this paper, a hybrid approach is proposed in which Finite Element Modeling (FEM) is used to simulate different loading conditions to obtain deformation data and in-turn, to obtain the simulated shearographic fringes. Different types of defects are embedded on the FE models and corresponding shearographic fringes are predicted. Correlating the defect and loading type to the predicted fringe pattern, in real shearographic measurements, different fringe patterns can be interpreted and classified. Also, camera calibration and image registration algorithms are used to project shearographic data onto the sample itself to locate and visualize the position of defects.

The most serious stresses are often at the edges of geometric discontinuities, and thereby influencing the overall performance of a structure. Under adiabatic and reversible conditions, thermoelastic stress analysis (TSA) provides nondestructive full-field information of the first stress invariant in a cyclically loaded structure. However, TSA measurements at and near the edges of discontinuities, the regions of prime interest, are often unreliable due to the adverse influence of the surrounding ambient temperature as well as the movement associated with the cyclic loading. Moreover, TSA data at such locations are susceptible to nonadiabaticity because of high stress gradients, thus further supporting the need to predict stresses at edges. A method is presented here for correctly quantifying the often disregarded TSA data at the edges of a structure by making use of the linear elastic conditions of equilibrium and compatibility as well as applying the appropriate boundary conditions. The method is hybrid in the sense that experimental TSA data (excluding disregarded edge measurements) are combined with an analytical expression of the first stress invariant. The achieved improvement in thermoelastic data near discontinuities is demonstrated here for a tensile aluminum structure containing a central irregularly shaped cutout.

Infrared thermography (IR) was used in this work that aims to experimentally evidence the stress network in granular media composed of two materials featuring different stiffness, without cohesion and under confined compression. Cylinders of polyoxymethylene (POM) and high-density polyethylene (HDPE) were used to build 2D composite granular systems. Cylinders were placed parallely and mixed together in a square metallic frame. The experiments were performed using a uniaxial testing machine. The granular media were first compacted in order to reach static equilibrium configurations. A cyclic compressive load was then applied. IR camera was employed to measure the temperature changes due to thermoelastic coupling on the cylinder network cross-sections. Temperature variations were then processed to obtain the maps of the amplitude of the sum of the principal stresses during the cycles. Three configurations were tested by changing the ratio between the POM and HDPE diameters and the ratio between the numbers of POM and HDPE cylinders. The experimental technique enables us to identify the stress network within the granular media. The experimental results are compared with numerical results obtained with a molecular dynamics software.

An experimental protocol was developed to achieve complete energy balances associated with low cycle fatigue (LCF) of a polyamide 6.6 matrix (PA6.6). The protocol involves quantitative infrared techniques (IRT), and digital speckle image correlation (DIC). IRT data were used with a local heat diffusion equation to estimate strain-induced heat sources, namely dissipation and coupling sources, while DIC enabled strain and stress assessments. Both techniques were then successfully combined to quantify deformation, dissipated and stored energies and then to estimate the Taylor-Quinney ratio that is widely used in plasticity.

In this paper, the effects of loading frequency and relative humidity were investigated. It was shown that an increase of relative humidity resulted in a decrease in the mean stored energy rate per cycle, while the stored energy ratio was much smaller at low than at high loading frequency. In addition, it was found that this ratio could be negative at the last fatigue stage, just before macroscopic crack inception. These energy properties will act safeguards for the future development of a thermomechanical model of PA6.6 matrix behavior.

Friction Stir Welding (FSW) is a relatively new welding process, which was developed at The Welding Institute (TWI), United Kingdom, in 1991. FSW is a solid-state joining process, i.e. no melting occurs. The welding process is promoted by the rotation and translation of an axis-symmetric non-consumable tool along the weld centreline. Thus the FSW process is performed at much lower temperatures than the conventional fusion welding. Nevertheless the control of the temperature field is fundamental to guarantee a high quality joint. In the present work the temperature field during the welding process was measured using an infrared camera. The test was conducted on 6 mm thick 5754 H111 aluminium alloy plates, in bead on plate configuration, with constant tool rotation rate and feed rate. Furthermore a finite element model was implemented and validated on experimental measurement data to evaluate the temperature field also into the plate.

This work aims at studying the dynamics of strain localization associated with Lüders instability in mild steel under monotonic, uniaxial tensile testing using full field measurement techniques such as infrared thermal imaging and digital image correlation. Based on the spatiotemporal evolutions of temperature and strain an enhanced understanding on the Lüders deformation behavior is achieved by addressing band nucleation and propagation, band growth mechanism, inhomogeneity in stress, strain and strain rate distributions within the band and the thermomechanical coupling associated. The experimental results are also compared and explained with some of the proposed models and concepts on Lüders instability.

Exposed to mechanical stress, semiconductor materials may phase transform, resulting in changes of crystallographic structure and material properties, rather than deform by plastic flow. As a consequence, prediction of the state and distribution of strain in semiconductors has become crucial for the evaluation of performance and reliability of structures made of these materials. Indentation-induced phase transformation processes were studied by in situ Raman imaging of the deformed contact region of silicon, employing a Raman spectroscopy-enhanced instrumented indentation technique (IIT). This is, to our knowledge, the first sequence of Raman images documenting the evolution of the strain fields and combined changes in the phase distributions of a material under contact load.